Non-imaging tracking tools and method for hip replacement surgery

ABSTRACT

The invention includes a method of determining a surgical patient&#39;s pelvic position and inputting that position into a computer via a tracking system, suitable for use in navigating partial or total hip replacement surgery. The patient is first aligned with anatomical reference points in relation to corresponding locating features on a patient positioner. The positions of index features on the patient positioner are then acquired via a tracking system. Based on the positions of the index features and their known relationship to the locating features, the locations of the anatomical reference features are calculated and a pelvic plane is defined therefrom. The invention also includes a surgical tool for mounting a trackable target to a human bone, suitable for fixation to a human femur. This tool includes a removable bone clamp and a releasable coupling member, integrated with said removable bone clamp.

This application is a divisional of and claims priority of U.S.application Ser. No. 10/637,304 filed on Aug. 8, 2003 now U.S. Pat. No.8,002,772 in the United States Patent Office.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to computer assisted surgery generally and morespecifically to computer assisted total hip replacement (THR) or hiparthroplasty operations.

2. Description of the Related Art

Total hip replacement or arthroplasty operations have becomeincreasingly common in the United States, with more than 300,000 suchoperations occurring annually. Many of the procedures will eventuallyrequire revision, due to one of any number of problems. Problems canarise with the implant, which can wear, degrade or even fracture. Inother cases, dislocation of the replaced hip can occur, causing extremepain (not to mention inconvenience and expense). The incidence ofdislocation has remained at approximately 2-6 percent, in spite ofimprovements to technique and materials.

It is known that the incidence of post-surgical dislocation is relatedto the orientation of the hip replacement components, particularly tothe angular orientation of the acetabular shell component in relation tothe bony anatomy. See Lewinnek et al., “Dislocation after totalhip-replacement Arthroplasties,” Journal of Bone and Joint Surgery, Vol.60A, No. 2, PP. 217-220 (1978). The head and neck geometry of theimplant is also thought to be a factor.

In spite of the published research, the typical surgeon has not adoptedany sophisticated method of navigating hip replacement surgery, in spiteof the availability of several techniques. The most prevalent method isto rely on an acetabular impactor tool with a handle placed at an anglepredetermined so that if the handle is maintained at a level, horizontalorientation, the acetabular shell will be at a desired angle. Thismethod fails to consider the considerable movement and variation in thepatient's pelvic position during surgery; at worst it aligns the shellwith the operating table (not necessarily the pelvis). Moretechnological methods have been developed, including the sophisticatedmethod described in U.S. Pat. No. 6,205,411 (and related applications)to DiGioia et al. (2001). The method of DiGioia is an advance over theprior methods (which he summarizes authoritatively in his “Background”section).

DiGioia's method begins with extensive pre-operative imaging, includingrelatively expensive CT scanning. The pre-operative imagery is theninput into a digital computer model, which performs extensive,three-dimensional modeling including range of motion simulations of thepatient's anatomy in relation to a specific computer model of aparticular implant. Next, in an intra-operative phase, the pre-operativemodels are registered with intra-operative optical tracking data: a verylarge number of points are sampled on the pelvis and femur, and thecomputer fits the data to the pre-operative model. Finally, the implantis positioned to align as closely as possible with the optimizedcomputer model.

The method of DiGioia et al. is complex and requires sophisticateddigital and radiological techniques. A need still exists for a simplermethod of surgical navigation which will facilitate proper hip geometrywith a minimum of pre-operative imagery and expense. It is frequentlyfound that physicians are loath to adopt any methods, and particularlyany computerized methods, which are unduly complex, expensive or timeconsuming. In this they may be forgiven, in light of the increasingeconomic constraints which burden the modern practice of medicine.

Thus, a need persists for an intra-operative computer assisted hipnavigation system which is easily learned, rapidly executed,economically practical, and independent from expensive or exoticpre-operative radiological imagery.

Furthermore, there is a need for specific methods and apparatus whichwill facilitate tracking a patient's pelvic plane and a patient's femur,in connection with a computer assisted hip navigation system.

SUMMARY OF THE INVENTION

In view of the above problems, the present invention includes a methodof determining a surgical patient's pelvic position and inputting thatposition into a computer via a tracking system, suitable for use innavigating partial or total hip replacement surgery. According to themethod, the patient is first aligned with anatomical reference points inrelation to corresponding locating features on a patient positioner. Thepositions of index features on the patient positioner are then acquiredvia a tracking system. Based on the positions of the index features andtheir known relationship to the locating features, the locations of theanatomical reference features are calculated and a pelvic plane isdefined therefrom.

The invention also includes a surgical tool for mounting a trackabletarget to a bone, suitable for fixation to a human femur. The toolincludes a removable bone clamp and a releasable coupling member,integrated with said removable bone clamp. The coupling member isarranged to mate with a compatible coupling member mounting thetrackable target to establish a predetermined spatial relationshipbetween said bone clamp and said trackable target. Furthermore, thecoupling member includes a releasable connection between said target andsaid bone clamp, to remove said trackable target. The connection iscapable of re-engaging in a re-engaged position which accuratelyrecaptures said spatial relationship between the bone and said trackabletarget.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system-level block diagram of the environment in which theinvention operates;

FIG. 2 is an exploded perspective view a trackable femoral marker whichattaches to a femur via a clamping device, in accordance with theinvention;

FIG. 3 is a perspective view of the trackable femoral marker of FIG. 2,fixed in a typical position on a human femur;

FIG. 4 is a front view of the bone clamp portion of the trackablefemoral marker, with trackable target removed and releasable couplingseparated;

FIG. 5 is a top view of a dovetailed first member of the releasablecoupling, useful for removably attaching the bone clamp to the trackabletarget;

FIG. 6 is a side view of the dovetailed first member shown in FIG. 5;

FIG. 7 is a top view of a second member of the releasable coupling,capable of mating with the first member shown in FIGS. 5 and 6;

FIG. 8 is a front view of the second member shown in FIG. 7;

FIG. 9 is a side (end) view of the second member shown in FIGS. 7 and 8;

FIG. 10 is an isometric view of a calibrated patient positioner, usefulfor locating a patient's pelvic plane in accordance with the invention;

FIG. 11 is a top view of the calibrated patient positioner of FIG. 10;and

FIG. 12 is a flow diagram of a method for determining a patient's pelvicposition and inputting that position into a computer via an opticaltracker.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a system-level block diagram of a system or apparatus 20which provides the environment in which the present invention operates.The system or apparatus 20 is generally a computer aided system fornavigating orthopedic surgery. A physician or other professional 22performs a hip surgery (for example, total hip replacement) on a patient24. An optical or equivalent locator or locating system 26 is disposednear the patient, so that the operating field is encompassedsubstantially within the field of view 28 of the locator 26. A suitableoptical locator is available commercially, for example the “Polaris”available from Northern Digital Inc., in Waterloo, Ontario, Canada.Optical trackers or markers 30 are used during the operation, as morefully described in related application U.S. Ser. No. 10/075,796. Themarkers 30 allow the locator 26 to acquire the positions andorientations of tools and anatomical reference points, as describedbelow.

The optical locator 26 is interfaced with and outputs tracking data to adigital computer 32, which interprets the optical tracking data as it isreceived. Using well known geometric relationships, the computer isprogrammed to deduce from the optical field of view the actual positionsand orientations of the markers, and, by extension, the positions andorientations of the instruments and/or anatomical features that are inknown relationship to the markers. For example, suitable optical markersutilizing multiple reflective spheres are available from TraxtalTechnologies in Toronto, Ontario, Canada. Markers with active lightemitting devices such as LEDs are also available and could equivalentlybe used. Note that typical markers include three or more non-collinearcomponents; this allows the locator and computer to determine not onlythe positions but the orientation (rotation) of such a marker in space.This capability is exploited in the methods described below.

Preferably, the computer 32 is also programmed with a user-friendlyinterface (software) which facilitates the execution of the method ofthe invention (described below in connection with FIG. 2). The physicianor other personnel can view output (for example on a video monitor) andinput instructions to the computer 32 via I/O devices 34, which suitablycould include a monitor, keyboard, printer, foot pedals, and otherinput/output devices such as conventional “mouse” or similar pointingdevices.

Preferably, the system also includes a record storage device 36 such asa CD-R drive, and/or simply a printer which prints out a summary of theoperation and patient data for future reference or medical archiving.

Methods for computer assisted surgical navigation during hip replacementsurgeries are described in other patent applications. See, for example,U.S. application Ser. No. 10/075,796 incorporated herein by reference.

The present invention provides apparatus and method for tracking a femurand a patient's pelvis, suitable for use in connection with anycompatible computer assisted hip navigation system. The methods andapparatus described and claimed are useful in the context of the relatedapplications (for example, Ser. No. 10/075,796) but are not limited intheir applicability to the exact methods of those applications. Thepresent apparatus and methods are useful in any context in which it isdesirable to accurately and conveniently track the position and/ororientation of a patient's femur or pelvis.

The apparatus of the invention is fixable to the femur in a firm andfully engaged position which does not allow slippage or rotation, butwithout the use of bone screws, pins or any other bone damaging devices.Specifically, in accordance with the invention a trackable marker isattachable to the femur by a device which does not penetrate the outercortical (hard) shell of the bone. It is permissible, in accordance withthe invention, to use aggressively textured surfaces, which couldinclude spikes or cleats which do not fully penetrate the outer corticalshell. Specifically, it is important that the device should not intrudeinto the upper femoral canal. The inventors have determined that bonescrews or pins are not suitable for attaching a marker to the upperfemur during hip replacement surgery. Any such penetrating screws orpins would interfere with preparation of the femoral canal and insertionof a trial or permanent hip stem. Furthermore, penetration into the bonewould reduce the structural integrity of the bone. The structuralintegrity of the greater trochanter, for example, should not becompromised; this structure is subjected to high stress due to thebiomechanics of the hip joint.

In accordance with a first aspect of the invention, a femoral trackingmarker includes a fixing device for affixing a trackable target to ahuman femur. One embodiment is shown in FIG. 2. The tracking marker(generally at 50) comprises: a removable bone clamp 52; and a releasablecoupling 54, integrated with or fixed to the bone clamp 50 and arrangedto mate with a compatible coupling 56 on a trackable target 58.Preferably, the trackable target is connected to the coupling 56 by anelongated stem 59, which need not be straight as shown. The coupling of54 and 56 permits releasable connection between the target 58 and thebone clamp 50 in such a way that a predetermined spatial relationship isestablished and re-established between the bone clamp and the trackabletarget 58 whenever the coupling is in mated position, notwithstandingany disconnect/reconnect cycles of the coupling. By extension, providedthat the bone clamp remains fixed vis a vis the clamped bone, aninitially established spatial relationship between the clamped bone andthe trackable target can be re-established, notwithstanding any numberof connection and disconnection cycles of the releasable coupling. Thisallows a surgeon freedom to remove the target for convenience, thenreattach the target and resume bone tracking without loss of accuracy.

FIG. 3 shows the bone clamp portion 52 of the femoral tracking markerassembly 50 in operating position: fixed to a femur 60 by clamping aboutthe greater trochanter. This figure shows a suitable manner of mountingthe femoral fixing device on a human femur. Specifically, a first jaw 64is arranged to engage the anterior aspect of the greater trochanter 62;the second jaw 66 is arranged opposite, with bone interposed between thejaws.

The clamp 52 is shown with the marker and stem 59 removed (bydisconnecting the releasable coupling 54 and 56). One member 54 of thereleasable coupling can be seen at the top of the bracket; thecomplementary member 56 is associated with the stem 59 and marker 50 andthus is not visible in this figure.

The releasable coupling facilitates surgery as follows. During surgery,as described above, the femoral tracking marker 50 is initially clampedto a patient's femur via bone clamp 52, with a trackable target 58initially coupled to the clamp 52 by the releasable coupling (54 and56). A locating system tracks the femoral tracking marker 50 during aninitial geometry acquisition. The locating system also tracks thepatient's pelvis and relates the pelvis to the femoral tracking marker.Thus, an initial relationship between the femoral tracking marker andthe pelvis is captured and recorded or stored, corresponding to aninitial offset and leg length.

Once an initial geometry has been captured, the releasable couplingfeature (54 and 56) allows a physician to remove the target 58 and stem59 from the femoral tracking device, to gain more convenient surgicalaccess to the hip and femur. The bone clamp portion 52 of the femoraltracking marker remains securely fastened to the femur. The opticaltarget can subsequently be reattached to the bone clamp 52 via thecoupling 54 and 56, and the previous relationship between the target andthe bone will be accurately and reliably reestablished. Reliabletracking of the femur can then resume (for example during surgicalnavigation steps in a hip replacement surgery.

As shown in FIG. 4 (and in previous FIG. 2), the bone clamp includes twoopposable, pivotable jaws: a first jaw 64 pivotable about a first axisX1 (constrained by pivot pin 72) and a second jaw 66 pivotable about asecond axis X2 (constrained pivot PIN 74). The first and second axes arepreferably constrained to be substantially non-parallel, and in fact theaxes X1 and X2 are most preferably constrained to be substantiallyperpendicular to one another. The two jaws are pivotably connected to aslidable, adjustable bracket 76, the extent of which is preferablyadjustable by some mechanism such as a tightening screw 78. The clampcan be attached to a bone by positioning the jaws 64 and 66, thentightening the bracket 76 by shortening its span with the adjustmentmechanism 78, thereby urging the opposed first and second jaws towardeach other to firmly clamp or pincer the bone as it is compressedbetween said jaws.

The adjustment mechanism 78 is shown as a screw in FIG. 4. One of thecomplementary members (54) of a releasable coupling (54 and 56) is alsovisible on top of the bracket 76, for mating the bracket with atrackable target.

The inventors have found that the arrangement of the jaws with two,substantially perpendicular pivot axes greatly facilitates secureclamping to an irregular bony surface such as that of the greatertrochanter. Secure purchase on the bone is also facilitated by grippingfeatures on the jaws. Preferably, each jaw has at least one (mostpreferably two or more) gripping features such as teeth, fangs, cleatsor a sharpened surface texture which tends to firmly engage with a bonesurface when pressure is applied between the jaw and the bone surface.However, it is greatly preferred that the gripping features be limitedin length to prevent the features from compromising the structuralintegrity of the cortical shell of the femur. Specifically, the grippingfeature should not intrude into the femoral canal, and preferably shouldnot fully penetrate the outer cortical shell of the femur. Two suchgripping features (or “teeth”) 84 and 86 are shown in the figure by wayof example.

The releasable coupling which couples the fixing device to an opticallytrackable marker has two complementary members: bracket member 54 andcomplementary member 56.

FIG. 5 shows the bracket member 54 of the releasable coupling (54 and 56collectively). This coupling reliably establishes a repeatablerelationship between trackable target 58 and the bone clamp 52. Adovetailed tongue 92 extends above the top of the bracket 76. Thedovetail shape can be seen in FIG. 6 (the end view). Top view of thedovetail shows that it is suitably formed within a partial cylinder, sothat the coupling member 54 when assembled with complementary member 56will together present a cylindrical form. A guide slot 94 is preferablyprovided to help center and guide the coupling by engaging a center pin96 carried in the complementary member 56 of the coupling (discussedbelow).

As shown in FIGS. 7, 8 and 9, a complementary (or “stem”) couplingmember 56 is fixed on the optically trackable marker. The stem member 56has a dovetail slot or void 100 which snugly receives and mates with thedovetail tongue 94 in the bracket member of the coupling. The dovetailslot 100 is preferably cut from a rotationally symmetrical piece(suitably of rigid material such as steel). Preferably, the slot is cutin a cylindrical piece; alternatively, the members 54 and 56 could bothbe cut from a conical volume. Generalized, the coupling members 54 and56 should preferably, when fitted together, describe a substantiallysolid joint which has rotational symmetry. Thus, when mated with thebracket member 54 the dovetailed tongue 92 occupies the dovetail slot100 so that the two complementary members together comprise asubstantially solid volume with rotational symmetry (for example, acylindrical or a conical volume). Once the coupling members are mated,the mating relationship is fixed and centered by lowering a sleeve 102to coaxially surround and center the two mated, dovetailed members. (Forclarity, the sleeve 102 is shown retracted and disengaged. It should beunderstood that the sleeve is slidable toward the right in the figure.)The sleeve 102 should preferably have rotational symmetry whichcorresponds to the exterior shape of the dovetailed joint. For example,a cylindrical sleeve 102 should have an interior dimension whichslidably fits and coaxially surrounds the two mated dovetailed members,causing them to tend toward a centered position. The upper portion 104of the sleeve 102 is suitably threaded with inside threads 104 whichengage with complementary threads 106 on an extension post). Rotatingthe sleeve causes the sleeve to slide over the dovetailed couplingmembers 54 and 56. The inside cylindrical diameter of the sleeve fitsthe outside of the cylindrical volume comprising members 54 and 56 andsecures the fit of 54 and 56 by containing the cylindrical volume,thereby securing the coupling in a locked and centered position.

Providing rotational symmetry for the dovetailed joint and sleeve isadvantageous in that it allows for ease of assembly, yet as the joint istightened it tends to center the assembly. The coupling is thusself-centering. This produces a reliable, well centered, repeatable fitwith little “slop” or error.

A center pin 108 can suitably be provided in the dovetailed slot 100 asshown in FIGS. 7, 8 and 9. This pin engages with a complementary slot 94in the dovetailed tongue member 92 (previously described and shown inFIG. 5). The center pin 108 and slot 94 facilitate rapid assembly of thecoupling by helping to align the dovetailed tongue 92 with thecomplementary dovetailed slot 100.

Optionally, an elongated stem 59 may used as shown to displace theoptical components of the trackable target 58 from the coupling (54 and56).

The fixing device according to the invention could alternatively bedescribed as a trackable target, capable of fixation to a bone,including: an adjustable bracket having first and second ends and anadjustment mechanism connected to adjust the displacement between thefirst and second ends; at least two jaws, one connected to each end ofthe adjustable bracket; a releasable coupling integrated with thebracket; and a trackable member having a compatible coupling which iscapable of mating with said releasable coupling in a predictable andrepeatable position and orientation. The optically trackable member mayoptionally be displaced from the coupling and clamp by a substantiallyrigid stem or other member, which need not be linear in form.

A further aspect of the invention is a Calibrated Patient Positionerwhich is optionally an integrated component of the navigation system. Byway of background, it should be understood that hip replacement surgerymay be performed with the patient supine (on his or her back) or in thelateral decubitus position (on his or her side). When the patient is inthe lateral position, certain pelvic landmarks may be obscured bysoft-tissue, draping, or conventional patient holders. Specifically,these landmarks are known as the pubic symphysis and the (right andleft) Anterior Superior Iliac Spines (ASIS). Lack of access to theselandmarks makes it difficult for the physician to palpate the pelviclandmarks during the acquisition of initial pelvic geometry.

The present invention provides an alternative to direct palpation tolocate the pelvic landmarks. In accordance with the invention, thepatient is initially clamped into a calibrated patient positioner whichhas specific features to “locate” the pelvic landmarks. Instead ofpalpating the pelvic landmarks directly, a calibrated trackable probe isused to touch distinct points on the positioner itself; given thepositions of these distinct points, software extrapolates the positionsof the pelvic landmarks. In this way, the calibrated probe need nottouch the patient at all; only the positioner must be in contact withthe patient.

FIGS. 10 and 11 show one embodiment of the positioner generally at 120.The device consists of a flat plate 122 preferably perforated with anarray of holes 124. Two clamps (not shown) are used to secure the plate122 to the operating room table. A Back Support Assembly 130 is disposedto support the patient from behind. The back support assembly includestwo tower rods 132 and 134, a cushion 136, and a height adjustment andlocking mechanism 138. In front of the patient is attached the FrontSupport Assembly 140, which consists of two tower rods 142 and 144, twoASIS cushions 146, a pubic slide body 148, and a height adjustment andlocking mechanism 150.

During set-up for surgery, the patient would be positioned on theoperating room table on their side on top of the plate 122. The BackSupport Assembly 130 would be inserted into the plate and the patientwould be held against the back cushion. The Front Support Assembly 140would then be inserted and adjusted until the two ASIS's were capturedin the two ASIS indicator concavities 160. The pubic slide body 148 andretractable pubic indicator 162 would be then adjusted until the pubicindicator, once extended and locked into place, palpates the pubicsymphysis of the patient.

The patient would then be draped and prepared for surgery. The hip jointwould be exposed in normal fashion and a pelvic tracker would beattached with a bone screw. After the navigation system is launched, thecomputer system (32 in FIG. 1) will prompt for palpation of theCalibrated Patient Holder. Using a trackable probe, index points such as171, 172 and 173 are palpated on the top ASIS cushion assembly 146.Whatever points chosen should have a previously known spatialrelationship to the ASIS indicator concavities 160. These index pointsare then used to define the body coordinate system (x, y, z). Becausethe top ASIS indicator notch is a fixed, previously known distance fromthe origin of this coordinate system, the location of the ipsilateralASIS can be calculated. Point 174 is then palpated and used to calculatethe location of the pubic symphysis, based upon a known (or directlymeasurable) spatial relationship between point 174 and the pubicindicator pointer 162. Because the system assumes symmetry of the pelvisabout the Y axis, the contralateral ASIS can be readily calculated. Oncethe location of both ASIS's and the pubic symphysis are known, theAnterior Pelvic Plane is defined and surgical navigation is enabled. Atthis point in the procedure, and before releasing the patient from thepatient positioner, the locating system and computer will locate thepelvic tracking marker and calculate the relationship between the pelvicmarker's orientation and the pelvic plane (as defined with the aid ofthe patient positioner).

After palpation of point 174 and acquisition of the pelvic marker'sorientation, the handle on the pubic indicator can be retracted toremove pressure from the pubic symphysis area. Indeed, after attaching apelvic tracking marker and acquiring initial geometry, the patient canbe released from the frame for repositioning during surgery. The pelvictrackable marker and optical tracker will allow the computer to trackthe position of the pelvic plane, notwithstanding movement of the pelvisduring surgery.

Software should be designed with appropriate provision for input orstorage of the spatial relationships between the pelvic landmarks andthe indexing points or features (points 171-174) on the positioner. Thepelvic landmarks will be constrained or located directly by theindicator concavities and the pubic indicator pointer. The relationshipsbetween these pubic landmarks and the index points should be madeavailable to software either by some form of input or by rigidlypre-determining the spatial relationships to dimensions stored asconstants. For example, the relationships between the index points andthe pelvic landmarks can be fixed by a rigid framework with knowndimensions. The known dimensions can then be assumed as constants incomputational algorithms.

The patient positioner is useful in a method of determining a patient'spelvic position and inputting that position into a computer via anoptical (or similar) tracker. Steps of such a method are shown in FIG.12. The patient is first aligned or positioned in the patient positioner(step 200) as described above, securing the ASIS cusions, ASIS indicatorconcavities and pubic indicator in contact, with the correspondingpelvic landmarks. The index points 171-174 are then acquired (step 202)suitably by an optical tracker, either by touching the points with atrackable probe or by tracking features mounted directly on the patientpositioner at known positions. The computer 32 then calculates (step204) the positions of the pelvic landmarks by compensating for (known)displacements between the pelvic landmarks and the optically capturedindex points 171-174. Based on the pelvic landmarks, a reference pelvicplane is then defined (step 206).

In some embodiments, the positions of the index points are capturedindirectly by touching the index points individually with a trackableprobe. Accordingly, the step of acquiring the positions of a pluralityof index points will include, in such embodiments, several steps:touching the index points with the probe; calculating the positions ofthe trackable probe while the probe is touching the index points; andfinally, compensating for a known dimension and shape (geometry) of thetrackable probe.

After calculating the pelvic plane, an optically trackable pelvic markeris then fixed to the patient's pelvis (step 208, for example, by a bonescrew). The computer then determines and stores the spatial relationshipbetween the trackable pelvic marker and the pelvic plane (step 210).This can be done suitably by determining a reference frame defined bythe pelvic tracking marker (for example, of reflective optical targets)and comparing it to the pelvic plane (while the patient is stillpositioned in the patient positioner). From the two frames a computercalculates a coordinate transformation between the marker referenceframe and the pelvic plane (and/or the inverse transformation).

After determining the relationship between the pelvic tracking markerand the pelvic plane, the patient's pelvis can be freed from thepositioner (step 212). The pelvic plane is then indirectly tracked (step214) by tracking the pelvic tracking marker (which is fixed to thepelvis). The corresponding orientation of the pelvic plane is easilyobtained by applying (to the tracker orientation) the previouslycalculated coordinate transformation, to obtain the pelvic plane. Withan appropriate optical tracker as described above, coupled with modestcomputing speed, the pelvic plane can easily be dynamically tracked inreal time without significant time lag.

The Calibrated patient positioner and the method of using it to acquireand track a pelvic plane can advantageously be employed in connectionwith the computer assisted surgical methods as described elsewhere.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. In some operations the acetabular implantmight not be required, but the femoral navigation methods and apparatusof the invention are still applicable. The procedure may be repeated onboth sides of the body in a bi-lateral THR operation. Differentelastomeric straps, fibers, cords, mesh, wire, adhesives or ligaturescould be employed in connection with the femoral tracking marker device.The fixed pelvic marker could also be fixed by alternate methods such asclamps, pins or even adhesives. The method can be adapted to variousbody geometries and sizes, and indeed could even be adapted, with smallmodifications, for veterinary medicine. Tracking means other than butsimilar to optical could be substituted, such as radio, microwave,magnetic, sonic or ultrasonic tracking systems, provided that the systembe not so clumsy or bulky as to interfere with the surgicalmanipulations required. Accordingly, the word “tracking” as used hereinshould be understood to include methods other than optical, but is notintended so broadly as to encompass mere mechanical stereotacticframeworks or electromechanical stereotactic frameworks. The geometriesof the various tools and markers can be varied or modified toaccommodate different tracking approaches. Active or passive opticaltargets can be used on the tracking markers. Such variations andalternate embodiments are contemplated, and can be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

1. A method of determining a surgical patient's pelvic position andinputting that position into a computer via a tracking system, suitablefor use in navigating partial or total hip replacement surgery,comprising the steps of: aligning the patient in relation to a patientpositioning frame with pelvic anatomical features of the patientdisposed in secure mechanical relationship with corresponding locatingfeatures on said positioning frame; acquiring with a tracking system thepositions of a plurality of index points, wherein each of said indexpoints is respectively disposed at a previously known displacementrelative to a corresponding one of said locating features; and defininga pelvic plane by calculation from the acquired positions of said indexpoints, said calculation including: compensating for the previouslyknown displacement of each said index point relative to itscorresponding locating feature, thereby finding the locations of saidlocating features from the acquired positions of said index points; anddefining a plane from the calculated positions of the locating features,such that said pelvic plane is defined based on the locations of saidpelvic anatomical features disposed in said secure mechanicalrelationship with said corresponding locating features.
 2. The method ofclaim 1, wherein said pelvic anatomical features are the right AnteriorSuperior Iliac Spine (ASIS), the left ASIS, and the pubic symphysis; andwherein said locating features comprise three locating features arrangedto engage said right ASIS, left ASIS, and pubic symphysis respectively,such that said three locating features define the pelvic plane.
 3. Amethod of determining a surgical patient's pelvic position and inputtingthat position into a computer via a tracking system, suitable for use innavigating partial or total hip replacement surgery, comprising thesteps of: aligning the patient in relation to a patient positioningframe with pelvic anatomical features of the patient disposed in securemechanical relationship with corresponding locating features on saidpositioning frame; acquiring with a tracking system the positions of aplurality of index points, wherein each of said index points isrespectively disposed at a previously known displacement relative to acorresponding one of said locating features; and defining a pelvic planeby calculation from the acquired positions of said index points, saidcalculation including: compensating for the previously knowndisplacement of each said index point relative to its correspondinglocating feature, thereby finding the locations of said locatingfeatures from the acquired positions of said index points; and defininga plane from the calculated positions of the locating features, whereinsaid pelvic anatomical features are the right Anterior Superior IliacSpine (ASIS), the left ASIS, and the pubic symphysis; wherein saidlocating features comprise three locating features arranged to engagesaid right ASIS, left ASIS, and pubic symphysis respectively, such thatsaid three locating features define the pelvic plane; and wherein saidlocating features include a retractable pubic indicator which isadjustable to palpate the patient's pubic symphysis.
 4. The method ofclaim 3, wherein said locating features further include at least oneASIS indicator concavity carried by a front support assembly of saidpatient positioning frame.
 5. The method of claim 4, wherein saidpatient positioning frame further comprises a rear support assemblyarranged adjustably to include the patient securely between said frontsupport assembly and said rear support assembly.
 6. The method of claim3, wherein said step of acquiring with a tracking system the positionsof a plurality of index points comprises: touching said index pointswith an optically trackable probe; calculating the position of saidoptically trackable probe while said probe is touching said indexpoints; and compensating for a known dimension and shape of said probe.7. The method of claim 1, comprising the further steps: fixing atrackable pelvic marker to the patient's pelvis; tracking the positionand orientation of said trackable pelvic marker while said pelvic planeis fixed in relation to said patient positioning frame; calculating aspatial relationship between the trackable pelvic marker and the definedpelvic plane; releasing the patient's pelvis from said patientpositioning frame; and tracking the pelvic plane indirectly by trackingthe position and orientation of said trackable pelvic marker thenapplying a coordinate transformation to calculate the pelvic plane, fromtime to time.
 8. The method of claim 7, wherein said trackable pelvicmarker is fixed to the patient's pelvis by a bone screw.
 9. The methodof claim 8, wherein said trackable pelvic marker comprises a marker witheither reflective or active trackable optical targets.